1. Field of the Invention
This invention relates to apparatus for cooling samples made of various devices and materials (such as various semiconductor devices, semiconductor materials, magnetic materials, superconducting materials, other metal materials or inorganic materials) and maintaining the samples at low temperatures when measurements, observations, or operations are performed regarding such samples at low temperatures reaching the boiling points of liquefied gases.
2. Description of the Related Art
Recently, high-sensitivity magnetometers having spatial resolutions on the order of micrometers and known as SQUIDs (Superconducting Quantum Interference Devices) have been put into practical use, and measurements using SQUID microscopes have been increasingly performed on various devices and materials. Since SQUIDs use superconductivity, it is necessary to cool them at temperatures lower than the temperature of liquid nitrogen (from several K to 77 K). Furthermore, the sample also needs to be retained at low temperatures in many cases. In addition, where samples are observed by tunneling microscopes or atomic force microscopes as well as by SQUIDs, samples are maintained at low temperatures in some cases.
FIG. 2 is a schematic view showing one example of cooling apparatus of related art for cooling a sensor side. A three-axis scanning stage 20, a cooling head 30, a coolant introduction port 42, a sensor 50, a sample 60, etc. are installed inside a vacuum chamber 10. A vacuum pump 70, a liquefied gas storage tank 40, and a transfer tube 90 are installed outside the vacuum chamber 10.
The vacuum chamber 10 is made of stainless steel and maintained in a vacuum state to make provide thermal isolation from the outside.
The three-axis scanning stage 20 is used to place the sample 60 and to control the relative position between the sensor 50 and the sample 60.
The cooling head 30 is a hermetically closed container made from oxygen-free copper to improve the thermal conduction. A first pipe 31 and a second pipe 32 forming an inlet and an outlet for the coolant are connected with the cooling head 30. The flow rate of the coolant flowing into the cooling head 30 is adjusted by a needle valve 33.
The storage vessel is a liquefied gas storage tank 40 which is a vacuum isolation container for storing a liquefied gas 41. Liquid helium is used as the liquefied gas 41.
The coolant introduction port 42 is used to introduce the liquid helium into the cooling head 30 installed inside a vacuum chamber. The coolant introduction port 42 and liquefied gas storage tank 40 are connected by the transfer tube 90, and the coolant stored in the liquefied gas storage tank 40 is introduced into the cooling head 30.
A SQUID having a detection coil about 10 xcexcm in diameter is used as the sensor 50. Niobium operating near the boiling point of liquid helium is used as a superconducting material for fabricating the SQUID. The sensor 50 is made stationary while placed in thermal contact with the cooling head 30.
The vacuum pump 70 is used to lower the pressure inside the second pipe 32, cooling head 30, first pipe 31, and transfer tube 90 and to transfer the liquid helium in the liquefied gas storage tank 40.
The procedure for cooling the cooling head 30 is as follows. The coolant introduction port 42 and liquefied gas storage tank 40 are connected by the transfer tube 90. The vacuum pump 70 is operated and thus the liquid helium stored in the liquefied gas storage tank 40 is passed through the cooling head 30. In this way, the temperature of the cooling head 30 is cooled close to the boiling point of liquid helium.
After cooling of the cooling head 30, the sensor 50 is operated, and the relative position between the sensor 50 and the sample 60 is controlled using the three-axis scanning stage 20. A signal owing to the sensor 50 is recorded. Thus, the magnetic distribution of the sample 60 is measured.
With the above-described cooling apparatus of the related art, where stored liquefied gas is directly used as means for cooling a sensor or a sample to a low temperature, the liquefied gas is often transported into a location to be cooled while using a thin pipe as a medium, or the liquefied gas is transported through a minute space such as a needle valve to adjust the flow rate of the liquefied gas. The stored liquefied gas often contains impurities such as solidified carbon dioxide, oxygen, nitrogen, and water, as well as foreign substances such as microscopic dust and metal fragments. Therefore, foreign substances and impurities sometimes clog up the pipe or needle valve that is a transportation medium for the liquefied gas. Consequently, there is a problem in that the apparatus ceases to function as cooling apparatus. Furthermore, where impurities adhere to the interface portion between the vacuum thermal isolation pipe and coolant introduction port, the interface portion becomes an adhesively bonded state. The vacuum thermal isolation pipe cannot be removed unless an operation for dissolving away the impurities is performed. Hence, the ending operation for the cooling apparatus cannot be performed. Thus, there is a problem in that the workability is poor.
(First Means)
In accordance with the present invention, a gas collection port is provided in a liquefied gas storage tank of a cooling apparatus. Gas produced by evaporation of the liquefied gas is collected and used as a coolant for a cooling head.
(Second Means)
In addition to the first means, a mechanism for measuring the liquid level of the liquefied gas is provided. The gas collection port is made movable vertically.
(Third Means)
In addition to the first means, a gas-cooling mechanism is provided.
(Fourth Means)
In addition to the first means, a structure is provided in which the liquefied gas storage tank is provided with a gas introduction port.
(Fifth Means)
In addition to the first means, a structure is provided in which a refrigerator and a gas introduction port are used instead of the liquefied gas storage tank.
According to the structure of the cooling apparatus owing to the first means, gas evaporated from the liquefied gas is used as a coolant for the cooling head and so even where impurities are mixed in the liquefied gas stored in the liquefied gas storage tank, a high-purity gas can be used as a coolant. Consequently, the pipe or needle valve for transporting the coolant is not clogged up. The cooling apparatus can be run stably.
Owing to the second means, the liquid level of the liquefied gas can be known. Therefore, the gas collection port can be placed close to the liquid level. Gas of lower temperature can be collected and used as a coolant. In consequence, the cooling head can be cooled to a lower temperature.
Owing to the third means, the collected gas becoming the coolant can be cooled to a lower temperature. As a result, the cooling head can be cooled to a lower temperature.
Owing to the fourth means, the pressure inside the liquefied gas storage tank can be adjusted. Therefore, the pressure inside the liquefied gas storage tank can be prevented from becoming a negative pressure. That the gas becoming the coolant cannot be transported can be prevented. Hence, the cooling apparatus can be run stably.
Owing to the fifth means, a high-purity gas can be used as a coolant without using a liquefied gas. Therefore, intrusion of foreign substances into the cooling apparatus can be prevented. The cooling apparatus can be run stably.